1. Field of the Invention
This invention relates to an alignment method and an alignment apparatus and is applicable, for example, to the detection of the position of an alignment mark used for the alignment of a mask and a wafer.
2. Related Background Art
An alignment mark detecting system 1 as shown in FIG. 6 of the accompanying drawings has heretofore been used in an exposure apparatus for manufacturing liquid crystal substrates or the like. This alignment mark detecting system applies a laser beam from a laser source 9 as alignment light to a substrate 2 placed on a stage through an optical system 10. By scanning the substrate 2 with the alignment light, the alignment light scans over an alignment mark 2a formed on the substrate 2, in the direction of arrow A.
Diffracted light reflected by the alignment mark 2a enters a detector 3 through the optical system 10 and is given as a received light signal S1 to a preamplifier 4. The gain of the preamplifier 4 is variably controlled by a gain control circuit 5, and the preamplifier 4 amplifies the detection signal S1 in conformity with a set gain and outputs an analog signal S2 of a suitable magnitude to an analog-digital conversion circuit (A/D) 6. The analog-digital conversion circuit 6 converts amplified signal S2 into a digital signal S3 and gives it to a waveform analyzing circuit 7.
The waveform analyzing circuit 7 detects the peak of the waveform (corresponding to the center of the alignment mark) from the digital signal S3, and calculates the coordinates of the center of the alignment mark on the basis of the coordinates information S4 of the substrate 2 obtained from a position detector 8.
The signal level of the analog signal S2 provided by the preamplifier 4 to the analog-digital conversion circuit 6 is not always of a magnitude suitable for the calculation of the coordinates of the center based on a single scan. So, heretofore, the alignment mark 2a has been twice scanned to thereby find the coordinates of the center of the alignment mark 2a.
That is, when the peak value of the signal waveform obtained from the digital signal S3 during the first scan is not an appropriate signal level, the waveform analyzing circuit 7 calculates again necessary to provide an appropriate peak value, and gives this gain as again signal S5 to the gain control circuit 5. It then calculates the coordinates of the center of the mark in such a manner that during the second scan, the peak value of the signal waveform becomes an appropriate signal level, and outputs them as center coordinates information S6.
This process of the waveform analyzing circuit 7, if represented by a flow chart, is such as shown in FIG. 7 of the accompanying drawings. That is, as a step SP1, the digital signal S3 of the signal waveform obtained by the first scan is entered, and at the next step SP2, the waveform is analyzed. The signal waveform at this time is shown in FIG. 8A of the accompanying drawings. The peak of the signal waveform is detected by the result of this analysis, and a gain for obtaining an appropriate signal level is calculated at a step SP3. Thereafter, at a step SP4, the gain of the preamplifier 4 is set to an appropriate value.
After these processes, the second scan is effected, and at a step SP5, the digital signal S3 of the signal waveform obtained by the second scan is entered. The signal waveform at this time is shown in FIG. 8B of the accompanying drawings. At a step SP6, the waveform of the analog signal S2, amplified by the gain obtained at the step SP3, is analyzed. At the next step SP7, the coordinates of the center of the alignment mark 2a are calculated from the result of this analysis.
However, in the above-described method, it is necessary to repeat the scan (that is, scan twice) each time the alignment mark is aligned, and this has led to the problem that for the entire substrate, the time required for alignment becomes long.